Uranium is normally present in the environment at a very low concentration, in the form of isotopes 238 (99.27%) and 235 (0.72%), which break down through the emission of alpha-particles of weak radiological toxicity; the uranyl form (UO22+) represents the most common form of uranium in an oxygenated atmosphere.
However, in specific sites, for instance close to uranium mineral mines, storage sites (nuclear control or storage of uranium-depleted munitions) or else in the case of a nuclear accident, the concentration of this metal may be much higher and represents a danger for humans due to the fact that it accumulates in the kidneys and in the bones: toxicity for renal tissue and development of bone tissue cancers.
The decontamination of contaminated sites and individuals requires having, firstly, means for neutralizing the toxicity of the uranium in the environment and in the body of the contaminated individuals and, secondly, detection reagents that are effective and specific for this metal. However, no effective means for detecting and decontaminating uranium currently exists, in particular due to the absence of uranium-specific ligands capable of detecting uranium (sensor) and of chelating this toxic metal which could be present in an environment and/or in a biological medium that is contaminated, in order to perform the decontamination thereof.
The current treatment for decontaminating soils is carried out mainly by excavation, harvest and storage in appropriate sites or by extraction of the uranium using chelating agents; these physicochemical treatments are expensive, not very specific or not specific at all, and not very suitable for the treatment of very widespread contaminating surfaces, and also present a high risk of contamination for the operators, due to repeated exposure to uranium. Alternatively, it has been proposed to use living organisms (higher plants or microorganisms) for decontaminating soils and water contaminated with uranium. This method is based on absorption of the metals and therefore on their sequestration by these organisms. For example, plants are capable of absorbing toxic metals by the roots and of accumulating them in the leaves, which are subsequently harvested and stored in appropriate sites; the organisms currently available do not allow effective decontamination of contaminated soils and water because of their low capacity for extraction, tolerance and accumulation of high concentrations of toxic metals.
The detection of uranium in individuals who may have been contaminated with uranium is carried out ex situ by plasma mass spectrometry (ICP-MS); this technique is laborious to carry out and expensive.
The treatment of individuals contaminated with uranium is carried out by the administration of chelating agents which bind the uranium, thus promoting its excretion and consequently reducing its deposition in the kidneys and the bones. Among the main uranium-chelating agents, mention may be made of: diethylene-triaminepentacetic acid (DTPA), 5-aminosalicylic acid (5-AS), gallic acid, sulfocatechol, carboxycatechol and hydroxypyridinone; these chelating agents have the drawback of not being uranium-specific.
Various approaches have been developed for specifically detecting certain metals, in particular in an aqueous medium or in biological samples:                fluorescent chemosensors (Tsien, 1993: Fluorescent chemosensors for ion and molecule recognition, pages 130-146, Czarnik A W (ed), American Chemical Society, Washington D.C.); these fluorescent sensors are specific for sodium, potassium, calcium and magnesium; on the other hand, no uranium-specific chemosensor has been described;        fluorescent peptide sensors (biosensors) consisting of a peptide of approximately 26 amino acids, derived from a zinc finger domain, labeled with at least one fluorescent group (Walkup et al., 1996, J. Am. Soc., 119, 3443-3450; Godwin et al., 1998, J. Am. Soc., 118, 6514-6515; Walkup et al., 1997, J. Am. Soc., 119, 3443-3540). In the presence of zinc ions, these peptide biosensors form a structure around the metal and expose the fluorescent group to environmental changes that result in a variation in fluorescence emission that depends on the concentration of the metal. Alternatively, when the peptides are conjugated to two appropriate fluorescent groups, the binding of the zinc to the peptide results in a conformational modification favorable to an effective transfer of energy between the two fluorophores (Fluorescence Resonance Energy Transfer or FRET), resulting in the emission of a fluorescent signal proportional to the concentration of the metal (Walkup et al., 1996, mentioned above). Because of the structure of the “zinc finger” domain, which is suited to the chelation of ions with a tetrahedral geometry, like zinc, these biosensors do not make it possible to chelate uranium, which, in its uranyl form (UO22+) which is the most common in an oxygenated medium, exhibits a pentagonal or hexagonal bipyramidal geometry with a coordination number of 7-8 (uranium VI);        fluorescent protein sensors, called “chameleons”, consisting of a fusion protein comprising successively, from its NH2 end to its COOH end: a blue or cyan mutant (EBFP or ECFP, fluorescence donor) of the GFP fluorescent protein derived from the jellyfish Aequorea victoria, calmodulin (CaM) comprising the N- and C-terminal domains and calcium ion-binding sites I and II, a calmodulin-binding peptide of 26 residues that is derived from the calmodulin-binding domain of a myosin light chain kinase (MLCK), and another green or yellow mutant of the same fluorescent protein (EGFP or EYFP, fluorescence acceptor) (Miyawaki et al., Nature, 1997, 388, 882-887). The binding of calcium to calmodulin causes a conformational change in the fusion protein, which forms a new site to which the peptide binds, and which produces an association between the two fluorescent proteins and a positioning in the space that is favorable to effective energy transfer from the fluorescence donor (EBFP or ECFP) to the fluorescence acceptor (EGFP or EYFP), thus producing an increase in the fluorescence emitted by the fluorescence acceptor (EGFP or EYFP). Other “chameleon” fluorescent indicators that are more sensitive and specific for a broader range of calcium concentrations have also been obtained (Truong et al., Nature Struct. Biol., 2001, 8, 1069-1073). This fluorescent indicator system, that is based on the conformational variations induced by the binding of calcium to the calmodulin-MLCKp complex, is calcium-specific and does not therefore make it possible to detect other metal ions, for instance uranyl;        peptide ligands selective for heavy metals, derived from helix-loop-helix motifs (Borin et al., Biopolymer, 1989, 28, 353-369; Dadlez et al., FEBS Lett., 1991, 282, 143, 146; Marsden et al., Biochem. Cell. Biol., 1990, 68, 587-601; Shaw et al., Science, 1990, 249, 280-283; Reid et al., Arch. Biochem. Biophys., 1995, 323, 115-119; Procyshyn et al., J. Biol. Chem., 1994, 269, 1641-1647); these peptides exhibit poor structuring of their helices in aqueous media, and also low affinities for divalent metals (Kd of the order of a millimolar);        bacteria containing a promoter that can reflect the presence of a toxic metal as a light signal (Bechor et al., Biotechnol., 2002, 94, 125-132; Lee et al., Biosen. Bioelectron., 2003, 18, 571-577); in these systems, the toxic agent acts as a cellular stress factor and thus induces an altered expression of a bioluminescent protein, which represents the detected signal; these systems are not therefore specific for uranium and for toxic metals in general.        
It emerges from the above that uranium-specific ligands capable of detecting (sensor) and of chelating this toxic metal which could be present in an environment or in a biological medium that is contaminated, in order to carry out the decontamination thereof, currently do not exist. The specific chelation of uranium in the environment (soil, water, etc.) and in living organisms is nevertheless difficult to carry out, due to the presence of a large excess of other metals, such as alkaline earth metals or lanthanides which are competitors for uranium-binding.
Consequently, the inventors have given themselves the aim of providing an agent capable of specifically chelating uranium (VI), in the uranyl form (UO22+)